1. Field of the Invention
The present invention relates to an image pickup apparatus having a high resolution and a high signal-to-noise (S/N) ratio using a confocal optical system, and a photomask defect inspection apparatus having such an image pickup apparatus.
2. Related Background Art
In a conventional photomask defect inspection apparatus, a photomask to be inspected has been scanned at a high speed by a light beam for illumination, a transmitted light through or a reflected light from the photomask has been received by a one-dimensional image sensor, and then output signals from the image sensor have been compared with data stored in a database or compared with each other to detect the presence of contaminants and defects of a light blocking pattern.
Another conventionally known technique for a defect inspection apparatus is that a transmitted light through or a reflected light from a photomask is received by a two-dimensional CCD camera, and output signals from light receiving elements of the CCD camera have been compared with data stored in a database or compared with each other to detect defects.
As integration and density of LSI become higher, a pattern of photomask becomes finer, thus placing strong demands for a photomask defect inspection apparatus with a higher resolution and defect detection signals with a higher S/N ratio. The above-mentioned technique of receiving a light from a photomask by a one-dimensional image sensor has the advantage of a relatively higher resolution because confocality is maintained. Charges stored in a light receiving element of the image sensor, however, is proportional to lighting time of an illumination light, that is, storage time of the charges. Accordingly, in the technique of high speed scanning using a light beam, time for a light from the photomask to enter to the image sensor is short, so an amount of the charges stored in the light receiving element become smaller, thus there are limits with regard to the S/N ratio of the defect detection signal.
On the other hand, in the technique of receiving a light from a photomask by a two-dimensional CCD camera, lighting time can be relatively longer, therefore a good performance can be obtained with respect to the S/N ratio. However, when taking a reflected image or a transmitted image of the photomask by the two-dimensional CCD camera, stray lights such as a flare or a glare enter to light receiving elements, thus there are limits to a resolution and to a defect inspection of a fine pattern.
Further, in order to optically inspect defects of a fine pattern, it is preferable to use a light having a short wavelength, that is, an ultraviolet light as an illumination light. However, with the ultra violet light, absorption by optical elements is large and sensitivity of a photodiode is low, there is a problem that a sufficient detection sensitivity is difficult to be obtained with a conventional defect inspection apparatus. Also, among types of defects are defects due to deposition of contaminants and pattern defects due to failure in accurate formation of a chrome light blocking pattern. If these types of defects can be distinguished, a defect inspection apparatus would be able to have wider applications.
A solution for the above problems is disclosed in U.S. Patent Publication No. US-2002-0044277-A1 (entitled: xe2x80x9cImage Pickup Apparatus and Defect Inspection System for Photomaskxe2x80x9d). In the image pickup apparatus according to the above related art, an illumination light is projected as a line of illumination light through a spatial filter having plural slits which extend in a direction perpendicular to a direction of movement of a sample. Also, a transmitted light or a reflected light from the sample is received by an image sensor through a spatial filter having plural slits which also extend in a direction perpendicular to a direction of movement of a sample. Therefore, a line confocal optical system is configured, and stray lights such as a flare or a glare are significantly decreased to be able to take images of a high resolution.
Further, in this configuration, a moving speed of the sample and a line transfer speed of the image sensor are linked with each other. That is, the moving speed of the sample stage and the charges transfer speed of the image sensor is set so that the time interval during which the sample moves from a position into which an illumination light having passed through an i-th slit of a first spatial filter enters to a position into which an illumination light having passed through an adjoining i+1th slit enters, and the time interval during which the image sensor transfers the charges stored in a line of light receiving elements into which a light having passed through an i-th slit of a second spatial filter enters to a line of light receiving elements into which an illumination light having passed through an adjoining i+1st slit enters become equal to each other. Accordingly, the same portion of the sample is illuminated plural times, and charges generated by each illumination is accumulated, thus noise can be greatly reduced to significantly improve a S/N ratio. Consequently, both resolution and S/N ratio can be simultaneously improved, and it is possible to provide a defect inspection apparatus with a much higher accuracy by using an image pickup apparatus having such a high resolution and high S/N ratio as an image pickup optical system of a photomask defect inspection apparatus.
In the embodiment shown in FIG. 3 of the above-mentioned prior art, a laser light source, a diffraction grating, and abeam deflection device are used as a reflection-type image pickup optical system. The number of sub beams produced by the diffraction grating corresponds to the number of slits of the second spatial filter arranged in front of a second image sensor for receiving a reflected light. Therefore, in order to irradiate the whole of the slits of the second spatial filter by the sub beams, it is necessary to have the sub beams scan across the entire width of the slits. Accordingly, the scanning time becomes longer, thus the prior art has a problem of being unable to comply with the demand for a higher scanning.
On the other hand, AO deflection element (an acoustic optical element) is often used for the beam deflection device. In the acoustic optical element, ultrasonic pressure waves are provided with crystal to diffract an incident light by a diffraction grating consisting of the pressure waves. A diffraction angle is determined by the speed of sound in the crystal and the grating constant fixed by a frequency of the ultrasonic given. If the frequency of the ultrasonic is changed here, a deflection angle is changed, thereby enabling scanning using the laser beam. The frequency is changed at a constant rate during the scanning, and since the change also advances in the crystal at the speed of sound, the deflection angle gradually changes depending on the position of the crystal in the direction to which the sound wave advances. This is sometimes called cylindrical lens effect. Due to this effect, the laser beam having done with scanning extends to be elliptical, which requires to be corrected. Also, a blanking interval which is a period during which the beam having done with scanning is moved back to the original angle at a high speed also takes a length of time required for the ultrasonic to pass through the crystal. The blanking interval takes about 7.5 micro second (xcexcs) for a crystal with an ultrasonic path of 5 mm, of TeO2 which can have a wide deflection angle. With high speed scanning performance of TDI (Time Delay and Integration) sensor having multitap output, it is easy to attain a image pickup speed of 6 xcexcs per 1 scanning line. It is not practical if such high speed scanning takes the blanking interval of as long as 7.5 xcexcs.
As in the foregoing, the conventional image pickup apparatus has the problem of taking long time for scanning using the laser beam, thus being unable to meet the demand for a higher scanning.
The present invention has been accomplished to solve the above problem, and an object of the present invention is thus to provide an image pickup apparatus enabling high speed scanning, and a defect inspection apparatus for photomask.
An image pickup apparatus according to the present invention is an image pickup apparatus having a sample stage moving sample whose image is to be taken in a first direction; a light source emitting a light beam; a diffraction grating generating mxc3x97n number of sub beams at predetermined intervals along a direction corresponding to the first direction and a second direction perpendicular to the first direction from the light beam; a beam deflection device deflecting the mxc3x97n number of sub beams in the first direction; an image sensor receiving a transmitted light or a reflected light from the sample, having plural light receiving elements arranged in a two-dimensional array along the first and the second directions, successively transferring the charges stored in each line of light receiving elements arranged along the first direction, line by line, at a predetermined line transfer speed, and successively outputting the charges stored in the light receiving elements; a spatial filter arranged between the sample stage and the image sensor, and having plural slits formed at a predetermined pitch along the second direction and extending in the first direction; and a drive control circuit controlling drive of the image sensor and the beam deflection device; wherein a reflected light or a transmitted light by the mxc3x97n number of sub beams scanning a surface of the sample enters into the image sensor through each slit of the spatial filter; the moving speed of the sample stage and the charge transfer speed of the image sensor are set so that the time interval during which the sample moves from a position into which an i-th line of the sub beam enters to a position into which an i+1-th line of the sub beam adjoining with the second direction enters, and the time interval during which the image sensor transfers the charges stored in a line of the light receiving elements into which a light having passed through an i-th slit of the spatial filter enter to a line of the light receiving elements into which an illumination light having passed through an adjoining i+1-st slit enters become equal to each other; and the drive control circuit controls drive of the beam deflection device so that irradiation areas formed by the movement of the mxc3x97n number of sub beams are to be continuous toward the first direction. In this configuration, a high resolution can be attained and the S/N ratio can be significantly improved; in addition, since the beam carries out scanning using the mxc3x97n number of sub beams, scanning can be completed in a short time. In a preferred embodiment, the drive control circuit controls drive of the beam deflection device so that irradiation areas formed by the movement of the mxc3x97n number of sub beams are to be continuous in the first direction in one scanning period.
Also, it is possible to have the beam scan more effectively when the drive control circuit controls drive of the beam deflection device so that irradiation areas formed by the movement of the mxc3x97n number of sub beams are to be continuous in the first direction during the time when a position into which an i-th line of the sub beam enters to a position into which an i+1-th line of the sub beam adjoining with the second direction enters.
Further, it is preferable that the drive control circuit moves the mxc3x97n number of sub beams in the first direction and also in the direction opposite to the first direction to carry out scanning. By scanning in this manner, serrated distortion of images can be reduced.
In a preferred embodiment, the beam deflection device is an acoustic optical element. The above mentioned image pickup apparatus is preferably used in a photomask defect inspection apparatus which inspects defects of a photomask.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.